The presence of gases on the surfaces of or contained within silicon wafers during the performance of coating or etching processes on the wafers is becoming a problem of increased importance in the manufacture of integrated circuits and semiconductor devices. As devices become smaller and the manufacturing process more precise, the presence of molecules of vapor on the surfaces that are to be coated or etched is emerging as a significant contributor to the production of defective devices. When coating and etching processes are performed, for example, by sputtering, any foreign particles on the surface being processed can result in a microscopic area that escapes the process. Accordingly, an essential circuit path or electrical characteristic of the device on which the affected area lies may be defective and the device unusable.
Gases such as water vapor are typically present on the surfaces of the wafers and are contained within the bodies of wafers when the wafers are introduced into processing machines, which typically contain a sealed vacuum environment. During the course of the coating or etching processes performed on a wafer, heat is often applied to the wafer or is generated by the performance of the process itself, or both. Sputter coating and etching processes, chemical vapor deposition process (CVD), and other physical or chemical coating and etching processes, performed with or without plasma, frequently require the wafer substrate to be elevated to temperatures of, for example, 500.degree. C. or 1000.degree. C. for the process to proceed optimally. Without subjecting the wafers to deliberate heating, the physical and reactive processing of the wafer surface usually generates heat that will elevate the temperature of the wafer itself to, for example, 600.degree. C. As a result, water molecules embedded in the silicone of the substrate may be released and may locate on the surface being processed, shielding an area of the surface from the coating or etching process, or reacting with the processing gasses.
It has been recognized that gasses and water vapor can be removed, at least in part, from the wafers by heating or baking the wafer at some elevated temperature in a vacuum for a specified period of time. This specified period of time may be many times longer than that required for the complete processing of the wafer in a particular apparatus.
In processing machines such as cluster tools, a plurality of different and randomly accessible processing modules are connected to a common wafer handling or transport module, in which the wafers are maintained continuously at a vacuum pressure level, and into and out of which the wafers are transferred to and from an external atmospheric pressure environment through one or more load-locks. In wafer processing, pretreating of the wafers to eliminate contaminates, to be effective, must take place after the wafers are sealed in the internal vacuum environment from which they will not be removed prior to processing. In such cluster tools, the preheating of wafers for the time necessary to rid them of gasses and other dissolved contaminants is a step that will often seriously reduce the productivity or through-put of the machine.
In U.S. Pat. No. 4,923,584, there is disclosed a provision for heating batches of wafers while held in load-lock or cassette modules of a wafer processing cluster tool, for the purpose of removing absorbed contaminants from the wafers. In such a device, a batch of wafers would be loaded into the load-lock through the load-lock door while the load-lock is opened to the atmosphere. When the load-lock is sealed and pumped to a vacuum, the batch of wafers is heated. When the heating is complete, the load-lock is opened to the interior of the cluster tool and the wafers are individually removed from the load-lock and one-by-one transported to the various modules for processing. In the device of this patent, two load-locks are provided, allowing one to be loaded or unloaded while the other is subjecting a batch of wafers to prolonged heating. The preoccupation of one of the load-locks of a two load-lock machine for prolonged preprocessing, however, deprives the machine of the loading and unloading flexibility for which the two load-locks were provided, resulting, in some cases, in delays in the operation of the machine.
Provision for batch heating in a load-lock, however, presents several problems. Industry standard cassettes for use in wafer processing cluster tools are usually made of a plastic such as polypropylene. Ideally, a cassette, preloaded with wafers, is positioned in the load-lock from the outside and sealed with the wafers in the load-lock. Such plastic materials will not, however, withstand the temperatures needed for effective preheating to remove gasses from the wafers. Therefore, the above identified patent proposes employment of a metal rack or holder within the load-lock. Such a rack must either replace the standard cassette or be loaded with the wafers from the cassette at the lock. Metal wafer holders, however, are, in some situations, prone to contaminate wafers as metal atoms propagate from the holder into the wafer at or near the points of where the rack contacts the wafers. In that the load-locks are usually loaded and unloaded manually, the use of fragile quartz racks is impractical. Furthermore, the use of a special rack in the load-lock precludes mere insertion into the load-lock of an industry standard cassette. This requires a separate loading and unloading step, either at the load-lock or at a separate external location.
Importantly, however, heating in a load-lock presents the problem of exposure of the heated internal structures of the load-lock to atmosphere, which increases the incidence of corrosion from oxygen contact and water vapor in the air. In addition, the opening of the load-lock when the contents are hot exposes persons loading and unloading the load-locks to the hazards of the high temperatures, a hazard that can be avoided at the expense of providing the machine with front-end robots, or wasting production time by allowing the load-lock to cool.
Notwithstanding the above efforts of the prior art, there remains a need to provide an efficient and effective method and apparatus for removing gasses and other absorbed contaminants from the surfaces and bodies of wafers in a sealed environment of a processing apparatus prior to the processing of the wafers, and to do so without introducing into the process the disadvantages and hazards of the prior art.